EDP Sciences
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Volume 380, Number 1, December II 2001
Page(s) 238 - 244
Section Formation, structure and evolution of stars
DOI http://dx.doi.org/10.1051/0004-6361:20011401

A&A 380, 238-244 (2001)
DOI: 10.1051/0004-6361:20011401

The long-period companions of multiple stars tend to have moderate eccentricities

N. Shatsky

Sternberg Astronomical Institute, Universitetskii pr. 13, Moscow 119899, Russia Royal Observatory of Belgium, Av. Circulaire 3, Bruxelles 1180, Belgium

(Received 20 April 2001 / Accepted 2 October 2001 )

We examined the statistics of an angle $\gamma$ between the radius vector of a visual companion of a multiple star and the vector of its apparent relative motion in the system. Its distribution $f(\gamma)$ is related to the orbital eccentricity distribution in the investigated sample. We found that for the wide physical subsystems of the 174 objects from the Multiple Star Catalogue $f(\gamma)$ is bell-shaped. The Monte-Carlo simulations have shown that our $f(\gamma)$ corresponds to the population of the moderate-eccentricity orbits and is not compatible with the linear distribution f(e)=2e which follows from stellar dynamics and seems to hold for wide binaries. This points to the absence of highly elongated orbits among the outer subsystems of multiple stars. The constraint of dynamical stability of triple systems is not sufficient to explain the "rounded-off" outer orbits; instead, we speculate that it can result from the angular momentum exchange in multiple systems during their early evolution.

Key words: binaries: visual -- stars: statistics -- formation

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